Why is blender not using all of my ram?
Simply because it doesn't need to use more.

My guess is that you assume that the rendering would be faster if used more memory. Sadly, it's untrue. And due to various bottleneck like memory latency, memory bandwidth (and probably more) : using more ram than it strictly need to use would probably slow down the rendering, not speed it up.

i have 32GB of RAM, my current rendering use ~32MB (which is ~1024  time than what it could use if it needed to).

EDIT : i'd like to add that RAM isn't "processing power", it's "storage"
 (temporary, and many order of magnitude faster than disk but storage nonetheless). In the same way, it won't go faster if you add more disk.